1. Statement of the Technical Field
The inventive arrangements relate generally to methods and apparatus for providing increased design flexibility for RF circuits, and more particularly for optimization of dielectric circuit board materials for improved performance.
2. Description of the Related Art
RF circuits, transmission lines and antenna elements are commonly manufactured on specially designed substrate boards. For the purposes of these types of circuits, it is important to maintain careful control over impedance characteristics. If the impedance of different parts of the circuit do not match, this can result in inefficient power transfer, unnecessary heating of components, and other problems. Electrical length of transmission lines and radiators in these circuits can also be a critical design factor.
Two critical factors affecting the performance of a substrate material are dielectric constant (sometimes called the relative permittivity or ∈r) and the loss tangent (sometimes referred to as the dissipation factor). The relative permittivity determines the speed of the signal in the substrate material, and therefore the electrical length of transmission lines and other components implemented on the substrate. The loss tangent determines the amount of loss that occurs for signals traversing the substrate material. Losses tend to increase with increases in frequency. Accordingly, low loss materials become even more important with increasing frequency, particularly when designing receiver front ends and low noise amplifier circuits.
Printed transmission lines, passive circuits and radiating elements used in RF circuits are typically formed in one of three ways. One configuration known as microstrip, places the signal line on a board surface and provides a second conductive layer, commonly referred to as a ground plane. A second type of configuration known as buried microstrip is similar except that the signal line is covered with a dielectric substrate material. In a third configuration known as stripline, the signal line is sandwiched between two electrically conductive (ground) planes. In general, the characteristic impedance of a parallel plate transmission line, such as stripline or microstrip, is equal to {square root over (Ll/Cl )}where Ll, is the inductance per unit length and Cl, is the capacitance per unit length. The values of Ll and Cl are generally determined by the physical geometry and spacing of the line structure as well as the permittivity of the dielectric material(s) used to separate the transmission line structures. Conventional substrate materials typically have a permeability of 1.
In conventional RF design, a substrate material is selected that has a relative permittivity value suitable for the design. Once the substrate material is selected, the line characteristic impedance value is exclusively adjusted by controlling the line geometry and physical structure.
Radio frequency (RF) circuits are typically embodied in hybrid circuits in which a plurality of active and passive circuit components are mounted and connected together on a surface of an electrically insulating board substrate such as a ceramic substrate. The various components are generally interconnected by printed metallic conductors of copper, gold, or tantalum, for example that are transmission lines as stripline or microstrip or twin-line structures.
The dielectric constant of the chosen substrate material for a transmission line, passive RF device, or radiating element determines the physical wavelength of RF energy at a given frequency for that line structure. One problem encountered when designing microelectronic RF circuitry is the selection of a dielectric board substrate material that is optimized for all of the various passive components, radiating elements and transmission line circuits to be formed on the board. In particular, the geometry of certain circuit elements may be physically large or miniaturized due to the unique electrical or impedance characteristics required for such elements. For example, many circuit elements or tuned circuits may need to be an electrical xc2xc wave. Similarly, the line widths required for exceptionally high or low characteristic impedance values can, in many instances, be too narrow or too wide for practical implementation for a given substrate. Since the physical size of the microstrip or stripline is inversely related to the relative permittivity of the dielectric material, the dimensions of a transmission line can be affected greatly by the choice of substrate board material.
Still, an optimal board substrate material design choice for some components may be inconsistent with the optimal board substrate material for other components, such as antenna elements. Moreover, some design objectives for a circuit component may be inconsistent with one another. For example, it may be desirable to reduce the size of an antenna element. This could be accomplished by selecting a board material with a relatively high permittivity. However, the use of a dielectric with a higher relative permittivity will generally have the undesired effect of reducing the radiation efficiency of the antenna.
An antenna design goal is frequently to effectively reduce the size of the antenna without too great a reduction in radiation efficiency. One method of reducing antena size is through capacitive loading, such as through use of a high dielectric constant substrate for the dipole array elements.
For example, if dipole arms are capacitively loaded by placing them on xe2x80x9chighxe2x80x9d dielectric constant board substrate portions, the dipole arms can be shortened relative to the arm lengths which would otherwise be needed using a lower dielectric constant substrate. This effect results because the electrical field in high dielectric substrate portion between the arm portion and the ground plane will be concentrated into a smaller dielectric substrate volume.
However, the radiation efficiency, being the frequency dependent ratio of the power radiated by the antenna to the total power supplied to the antenna will be reduced primarily due to the shorter dipole arm length. A shorter arm length reduces the radiation resistance, which is approximately equal to the square of the arm length for a xe2x80x9cshortxe2x80x9d (less the xc2xd wavelength) dipole antenna as shown below:
Rr=20xcfx802(l/xcex)2
where l is the electrical length of the antenna line and xcex is the wavelength of interest.
A conductive trace comprising a single short dipole can be modeled as an open transmission line having series connected radiation resistance, an inductor, a capacitor and a resistive ground loss. The radiation efficiency of a dipole antenna system, assuming a single mode can be approximated by the following equation:   E  =            R      r              (                        R          r                +                  X          L                +                  X          C                +                  R          L                    )      
Where
E is the efficiency
Rr is the radiation resistance
XL is the inductive reactance
XC is the capacitive reactance
XL is the ohmic feed point ground losses and skin effect
The radiation resistance is a fictitious resistance that accounts for energy radiated by the antenna. The inductive reactance represents the inductance of the conductive dipole lines, while the capacitor is the capacitance between the conductors. The other series connected components simply turn RF energy into heat, which reduces the radiation efficiency of the dipole.
From the foregoing, it can be seen that the constraints of a circuit board substrate having selected relative dielectric properties often results in design compromises that can negatively affect the electrical performance and/or physical characteristics of the overall circuit. An inherent problem with the conventional approach is that, at least with respect to the substrate, the only control variable for line impedance is the relative permittivity. This limitation highlights an important problem with conventional substrate materials, i.e. they fail to take advantage of the other factor that determines characteristic impedance, namely Ll, the inductance per unit length of the transmission line.
Yet another problem that is encountered in RF circuit design is the optimization of circuit components for operation on different RF frequency bands. Line impedances and lengths that are optimized for a first RF frequency band may provide inferior performance when used for other bands, either due to impedance variations and/or variations in electrical length. Such limitations can limit the effective operational frequency range for a given RF system.
Conventional circuit board substrates are generally formed by processes such as casting or spray coating which generally result in uniform substrate physical properties, including the dielectric constant. Accordingly, conventional dielectric substrate arrangements for RF circuits have proven to be a limitation in designing circuits that are optimal in regards to both electrical and physical size characteristics.
The invention concerns an antenna, formed on a dielectric substrate having a plurality of regions. Each region has a characteristic relative permeability and permittivity. First and second dipole radiating elements are formed on the substrate and define conductive paths. A reactive coupling element is interposed between the dipole radiating elements for reactively coupling the first dipole radiating element to the second dipole radiating element. At least one of the permittivity or permeability of a first substrate region coupled to the reactive coupling element is different respectively from at least one of a second relative permittivity and a second relative permeability of a second characteristic region of the substrate. Consequently, the first permittivity or the first permeability can be selected to provide a desired reactance value for the reactive coupling element.
In a broader sense, the invention can comprise any reactive element of selected value integrated within a circuit board substrate. In that case, the invention includes at least one conductive path defining a circuit element and selectively formed on first characteristic regions of a circuit board substrate. The substrate in the first characteristic region can have a first permeability and first permittivity. At least one reactive element can be interposed between portions of the conductive path. In particular, the reactive element can be formed on a second characteristic region of the substrate having a second permittivity and second permeability. Either the first permittivity or the first permeability (or both) of the first regions can be different respectively from the second permittivity and the second permeability of the second characteristic region of the substrate. Consequently, a desired reactance value for the reactive element can be determined at least partially by at least one of the second relative permittivity and the second relative permeability.
According to one aspect of the invention, the portions of the conductive path are adjacent end portions separated by a gap. In that case, the second characteristic region is preferably disposed between the end portions. Alternatively, or in addition thereto, a metal sleeve can be provided adjacent to the end portions for magnetic coupling. In that case, the second characteristic region can be disposed at least beneath the elongated metal sleeve.